(1) Field of the Invention
The field of the invention is that of photopolymer systems which can be crosslinked and/or polymerized by irradiation, and in particular by UV irradiation or by a beam of electrons, and which can be used in particular to form coatings, such as inks or varnishes.
These compositions are of the type comprising an organic matrix whose constituents bear functions that are reactive in polymerization/crosslinking, as well as a photoinitiator which, after absorbing energy, for example UV energy, releases a strong acid: H+ (cationic route) or a free radical (radical route). Cationic photoinitiators thus allow the initiation and propagation of cationic chain polymerizations, whereas radical photoinitiators (PIs) allow the triggering of chain polymerizations by formation of free radicals.
In certain applications of coatings or inks (or even of dark varnishes), additives are used, such as pigments, which are capable of appreciably increasing the viscosity of the compositions, to such a point that they can no longer be spread and manipulated. It goes without saying that these are unacceptable drawbacks for the applications under consideration. To overcome this, diluents have been incorporated into these compositions in order to reduce their viscosity.
In the case of clear varnishes, diluents or co-reagents are sought which are capable of improving the flexibility of the coatings, without reducing the reactivity of the systems.
The diluents most commonly used to date in photopolymerizable ink or varnish compositions are products of vinyl ether type (in particular divinyl ether) or of limonene type (in particular epoxidized limonene).
Although these products satisfy a certain number of specifications required in the technical specifications for the diluent, i.e., in particular, the miscibility with the organic matrix or the organic resin, the transparency or the inexpensive nature, these known diluents fail as regards considerations of environmental protection, safety and industrial constraints.
The reason for this is that these products are particularly volatile (relatively low flash point, of less than or equal to 110° C.), toxic, or even hazardous, since they are explosive and flammable. These known diluents, of the vinyl ether or epoxidized limonene type, are moreover irritants and sensitizing agents.
In addition, they do not make it possible to achieve sufficiently high rates of polymerization, which would allow the concentrations of photoinitiator to be reduced, thereby greatly easing the strain on the cost of the compositions.
(2) Description of Related Prior Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Photopolymerizable coating compositions comprising organic cycloaliphatic polyepoxides combined with an α,ω-epoxycycloaliphatic functionalized siloxane, present in an amount which allows acceleration of the crosslinking, are moreover known from patent application EP 0 389 927. The photoinitiators used are onium salts.
This composition is presented as having an improved speed of crosslinking. The α,ω-cycloaliphatic epoxy-functional siloxane is thus used as a polymerization accelerator for polymerizable coating systems, based on UV-polymerizable cycloaliphatic polyepoxides. That patent application discloses, in particular, compositions based on:                3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexylcarboxylate, which constitutes the polymerizable matrix,        an agent for reducing the energy required for the photopolymerization, said agent consisting of α,ω-3,4-epoxycyclohexyl-1-ethyldimethyldisiloxane, and        a photoinitiator consisting of (4-octyloxyphenyl)phenyliodonium hexafluoroantimonate as a solution in 2-ethyl-1,3-hexanediol.        
The drawback of compositions of this type is their turbid, coloured, and thus non-transparent appearance. They moreover contain a significant amount of metal, which is not foreign to the problem of absence of translucency and transparency and, what is more, is liable to disrupt the polymerization mechanism.
α,ω-Epoxy dimethyldisiloxanes are moreover known as monomers that can undergo cationic polymerization under UV, in the presence of photoinitiators of the onium salt type. The article by Crivello published in J. Polym. Sci.: Part A: Polym. Chem., 1994, Vol. 32, 683-697 deals with this subject and shows that the cycloaliphatic epoxide functions are more reactive in cationic polymerization under UV than conventional epoxide functions.
The article by R. P. Eckberg & K. D. Riding Polym. Mater. Sci. Eng., 1989, 60, 222-7 discloses the cationic polymerization/crosslinking, under UV activation and in the presence of a photoinitiator of the onium salt type, of mixtures of α,ω-epoxy dimethyldisiloxanes (MEME) with cycloaliphatic epoxides of formula: 
According to that document, the MEME has no pronounced effect on the efficacy of the rate of polymerization/crosslinking for MEME concentrations of less than 45% by weight relative to the MEME/CY 179 mixture (FIG. 1).
Such concentrations are not of a nature to promote the use of MEME, since this additive cannot be predominant in the formulation, since it would otherwise make the polymer/reticulate unsuitable for the applications for which it is intended (e.g. antiadhesive paper coating, varnish, inks).
What is more, the crosslinked products obtained from such MEME/CY 179 mixtures suffer from the major drawback of being brittle, which is unacceptable in a good many applications.
Finally, these compositions are also affected by turbidity and metal contamination defects, in much the same way as the compositions according to EP 0 389 927.
The unavoidable conclusion is that the prior art does not suitably satisfy the specifications expected of compositions that can be polymerized and/or crosslinked by irradiation, such as those intended for ink and varnish applications. In particular, the prior art does not disclose, or even suggest, means (diluents) for controlling the viscosity of photopolymerizable pigmented inks which are not toxic and which do not have a harmful effect on the reactivity of the composition.
Similarly, in the field of photopolymerizable clear varnishes, no inoffensive additives exist which are capable of improving the flexibility of the coatings, the spreading or the surface state, without at the same time impeding the photopolymerization and/or crosslinking.
It should also be noted that the prior art does not provide a satisfactory solution as regards, on the one hand, the problems of transparency and translucency, and, on the other hand, the presence of metal impurities, in the photopolymerizable compositions under consideration.
Finally, the prior art presents an even greater dearth in the case of polymerization reactive functions which are not epoxy but, for example, either alkenyl ether (e.g. vinyl ether)—via the cationic route—, or acrylate or hydroxyl—via the radical route.
In this state of knowledge, one of the essential aims of the present invention is to overcome the existing deficiencies by providing compositions that can undergo cationic and/or radical photopolymerization, which satisfy the expectations of their fields of application, in particular as regards inks and varnishes.
With this set objective, the Applicant Company has, to its credit, on the one hand, isolated and selected a silicone compound of the functionalized di- or oligosiloxane type, and, on the other hand, used this compound in a novel manner as a diluent in photopolymerizable compositions, for example in inks or varnishes.